An Observational History of Mars

by Dave Snyder
Printed in Reflections: April and May 2001
(updated January 2002).

This year the
“Astronomy on the Beach” at Kensington Metropark will
focus on Mars and the International Space Station (ISS). The ISS has
generated a lot of interest recently, but why focus on Mars? I suppose the
obvious answer is that Jupiter and Saturn are past their prime for this year and
we don’t have any bright comets to work with. However Mars is a
fascinating object and besides, this June Mars will be closer to the earth than
it has been at any time since 1988.

Mars has been observed by many ancient cultures - we have no idea who was the
first to notice it. Those who did, noticed a pale pink object that was
only visible in the early morning just before dawn (and rather difficult to see
at that). This object moved relative to the stars, got brighter over the
next year and rose earlier and earlier. Then it abruptly stopped and
reversed direction. At its brightest it was the third brightest object in
the night sky (only Venus and the Moon were brighter), had an intense red color
and was visible all night long. After moving the “wrong”
direction for some 70 days or so, it stopped and reversed direction
again. It gradually got dimmer, was only visible in the evening sky and
set earlier and earlier. After another year it again was a pale pink
object, this time only visible just after sunset. Shortly after that, it could
not be seen at all. It remained hidden for about one hundred days when the
cycle repeated again. Each cycle took a little over two years.

The Greeks named this object Ares after their god of war. The Romans in
turn named it Mars after the roman god of war.

The scientific study of Mars begins in 1600 when various individuals (mainly
scientists, but a surprising number of amateurs as well) observed the planet
with telescopes. Our knowledge increased gradually, but certain
misconceptions became established, many of which persisted until the last half
of the 20th century. It is then we start to see groups (instead of
individuals) studying Mars. Some of these groups were composed of
amateurs, some were composed of scientists and some were composed of both
amateurs and scientists. The most successful of these groups was
NASA. While NASA’s primary objective was landing men on the Moon, clearly
the second item on NASA’s priority list was the scientific exploration of Mars
using unmanned spacecraft.

NASA, along with its soviet counterpart IKI, sent numerous spacecraft to the
red planet. While there were many failures, there were many successes as
well. In addition, out of the thousands of meteorites that have been
examined by scientists, a dozen or so are now known to have originated on
Mars. The data from the spacecraft and the meteorites have dramatically
changed our picture of Mars and gave us a seemingly endless set of clues to the
geology, chemistry, atmosphere and possible biology of Mars. However in
the process we seemed to have created more questions than answers.

A detailed chronology follows:

356 or 357 BC: Aristotle observed Mars passing behind the Moon.
We now call such an event an occultation. This convinced Aristotle that
Mars was more distant than the Moon.

1609: Galileo Galilei makes the first telescope observation of Mars.
He cannot detect any surface detail, but he notices it is not perfectly
round. (We now know that Mars is 100% illuminated only near opposition, at
other times it is shaped like a gibbous Moon).

1659: Christiaan Huygens made the first useful sketch of Martian surface
features. (Modern images of Mars show a planet with dark red regions among
lighter red regions. In the following text I will use term
“maria” for the dark regions and the term “desert” for the
light regions. This matches modern usage, but these terms were not used in
1659. However do not assume the deserts are hot dry places. Maria
often appear greenish in color but we now know this is an optical illusion.)

With Huygens’ crude telescope, he saw one of these maria (which is now called
Syrtis Major). He observed it move that night and again the next
night. He concluded that Mars rotated on its axis with a rate of 24
hours. Huygens believed that Mars might be inhabited, perhaps even by
intelligent creatures. He shared that belief with many other scientists
who would observe Mars over the years to come.

1666: Giovanni Cassini conducts more careful observations. He
concludes the rotation rate is 24 hours and 40 minutes. While there is
some question on this matter, Cassini is probably the first to notice that Mars
has white spots located near the poles. For the next 300 years people
assume these spots are made up of snow, ice or both (we now call these spots
“polar caps”).

1719: Cassini’s nephew Giacomo Filippo Maraldi conducts some observations of
his own. After many years, Maraldi is convinced that the shapes of some
maria change over time. He thinks this is evidence of clouds that
sometimes obscure the surface. He also saw changes in the polar
caps. He speculates this showed evidence of seasons: ice from the polar
caps supposedly melted during the “summer” and freeze again during
“winter.”

1783: William Herschel confirms Cassini’s suspicions that Mars has
seasons. This is based partly on Cassini’s observations, partly on his own
observations, but also on the fact that Mars has an inclination that is close to
the same value as Earth’s.

Herschel seems to be the first to refer to the maria by the term
“sea,” however he was not the first to assume that maria actually
contained liquid water. He suggests that flooding may explain some of
surface changes, though he agreed that clouds could explain some changes.

1860: Emmanuel Liais suggests the variations in surface features are due to
changes in vegetation (not flooding or clouds).

1863: Father Pierre Angelo Secchi notices that maria change color. At
different times he observed maria with green, brown, yellow and blue colors.

1877: Since the Earth, Jupiter and Saturn were known to have moons,
scientists suspected Mars might have moons as well. However finding them
was not easy. Asaph Hall had been searching for Martian moons, but
he found nothing. He almost gave up, but his wife insisted he keep
trying. Soon, Hall was rewarded with two small moons, which
were given the names Deimos and Phobos.

Giovanni Schiaparelli makes a map of Mars that showed maria, some of which
were connected by thin lines. He wasn’t the first to observe them (earlier
maps show a few) but he saw more lines than his predecessors. However some
observers did not see any lines, and there was some controversy over whether
they existed at all.

Schiaparelli assumed that these lines were natural landscape features.
He gave them the name “canali” which is the Italian word for
“groove.” However when this word was translated into English,
“canali” became “canal,” a word with a very different
meaning. This simple mistake led many people to speculate about
intelligent beings who built canals. Schiaparelli himself was unconvinced
and somewhat annoyed that his observations led to such speculation. He did
not think the lines proved anything about life on Mars, though he remained open
to the possibility.

1892: Edward Emerson Barnard observed craters on Mars. This
observation was almost completely ignored for over 70 years.

Public attention was first drawn to the Martian canals, mainly through
the efforts of Schiaparelli and the French astronomer
Camille Flammarion. However Percival Lowell kept the canals in the public’s attention.
Lowell was born into a wealthy Massachusetts family and was well educated (he
graduated from Harvard). While he was aware of current astronomical theories,
he seemed more interested in other matters (which included travels to Japan).
He owned a small telescope, but there is no evidence he did any serious
observing with it. However, Lowell was well connected; among his numerous
acquaintances was the Harvard astronomer, W. H. Pickering. Lowell and Pickering
corresponded with each other on the subject of Mars.

1893: Lowell was given one of Flammarion’s books as a Christmas
present. This book discussed what was known about Mars, including the canals
and Flammarion’s own ideas, in particular the suggestion that the canals might
be signs of intelligent life. Lowell read the book and became obsessed with
Mars.

1894: Only someone with Lowell’s wealth and connections would take this obsession to
the next step. Lowell decided to build an observatory he could use to study the
red planet. He did not take the easy approach and build an observatory near his
home in Boston; rather he considered many possible locations in an attempt to
find the best seeing conditions. Seeing is a term used by astronomers; good
seeing means there is little or no turbulence in the atmosphere. Even though he
wasn’t the first to understand the importance of good seeing, it wasn’t widely
understood at the time and Lowell made a large number of people aware of it.

Lowell convinced Pickering to join him in a trip to Arizona to scout out
possible locations. Pickering brought his assistant, Andrew Douglass and
eventually the three of them set up an observatory near Flagstaff and conducted
systematic observations of the red planet. These observations gave Lowell a
well-deserved reputation as one of the best planetary observers.

Pickering left the observatory after a couple years, but Douglass stayed until
he was fired in 1901. That is when Douglass started doubting Lowell’s canal
observations. To fill the positions Lowell hired Vesto M Slipher, Carl Lampland
and Vesto Slipher’s brother Earl C Slipher as assistants. In 1902 Lowell was
appointed to the Massachusetts Institute of Technology as a non-resident
astronomer. He could have just continued with his observations and be
remembered as a skilled astronomer. However Lowell was not content to just
observe. He had numerous theories, some of which involved canals and the
intelligent creatures that supposedly built them. These theories were
internally consistent and very ellaborate.

1906: Lowell publishes a book “Mars and its Canals.” This
book was widely read by the general public and goes into detail on Lowell’s
ideas on the canals. He claims
the canals were built by
Martians for the purpose of transporting water from the poles to the dry Martian plains.

1907: Alfred Russel Wallace (a well known biologist) responded with a book
of his own in which he argued that Mars is completely uninhabitable.
Wallace used measurements of the light coming from Mars and argued that Mars has a
surface temperature of minus 35 degrees Farenheit. Lowell’s claim that
there was liquid water must be wrong. He also concluded that the polar
caps consisted of frozen carbon dioxide not water ice as Lowell and many others
had assumed.

1909: The available observations did not always support Lowell’s ideas. There was
growing doubt about the existence of the canals themselves, not to mention the
rest of Lowell’s ideas. When he encountered skepticism, Lowell became dogmatic
and found new audiences for his ideas by giving public lectures, writing
books and writing articles in popular magazines.
Lowell became an outcast in the scientific
community. However he had support from a few scientists. In particular,
Flammarion was always sympathetic to Lowell and his ideas.
He has become well known and respected by the general public.

Lowell’s activities discouraged many scientists, particularly
in the United States, from studying Mars as it no longer seemed a
“serious” subject worthy of scientific pursuit.
However in Lowell’s defense, some have argued that he deserves credit for
developing modern planetology, a word Lowell invented. He originated the
notion that the Martian climate has changed over time, a notion we now believe
to be correct. He insisted that any theory of planetary evolution needed
to account for changes in all the planets not just the planet a scientist
happened to be studying. And he was the first to suggest that Mars is the
best location to test theories of climate change. This might help
scientists studying changing in the Earth’s climate. In some respects
Lowell was almost a hundred years ahead of his time. On the other hand,
his ideas on
possible Martian biology seem antiquated to the modern observer.

1912: Svante Arrhenius has an alternate suggestion for the Martian surface
variation: Mars might be covered with salts, during the winter the salts
have a light color. When the polar caps melt in summer, the salts absorb
water and develop a darker color.

1938: On the day before Halloween, Orson Wells produces a radio production
of the fictional story “War of the Worlds.” This is a story of
Martians invading the earth. The production was so convincing, that many
people believe there has been a real invasion by Martians. A panic
resulted.

1947: The Association of Lunar and Planetary Observers (ALPO) is
formed. ALPO along with the BAA (which was founded in 1890) and other
organizations coordinate several Mars observation programs. In such
programs, amateur and/or professional astronomers from around the world pool
resources. Over the next several years, these programs provide several
extended periods of almost continuous observations (because you can observe Mars
only at night, it is impossible for a single observer at a single location to do
this - but a group of observers can).

1952: Gerard Kuiper makes the first attempt to determine the composition of
the Martian atmosphere using modern equipment. He discovered spectral
lines that indicated carbon dioxide. For several decades, researchers had attempted to measure the
atmospheric pressure of Mars. Estimates varied over a wide range, from less
than 24 millibars to well over 90 millibars (by comparison the earth’s
atmospheric pressure is about 1000 millibars). However scientists did not think there
could be 24, let alone 90 millibars of carbon dioxide. Therefore they reasoned the remainder of the
atmosphere was made up of something else.
This was assumed to be nitrogen and argon (since these were the only non-reactive gases likely to be present on Mars that
wouldn’t have been detected by the analysis methods used at the time).

The canal controversy would not be completely resolved until spacecraft arrived
at Mars.
In the 1960’s most scientists thought there were no canals on Mars, however
there were a few exceptions, such as Earl Slipher.
He wrote several books, some of which contained photographs.
Slipher claimed these photographs
had lines in the same place as the canals of Percival Lowell.
One of these books was published as late as 1964.
That same year, after a few U. S. and soviet failures, a U. S. spacecraft,
Mariner 4, is the first to flyby Mars. In 1969, Neil Armstrong walks on the
Moon. Some consider that a manned mission to Mars is the next step. However
there are problems with the idea. A round trip would take two years. Enough
fuel and water must be carried on board so the astronauts could survive and
return to earth. The weight of that fuel and water adds to the expense. A one
way manned trip to Mars (assuming one could find anyone to volunteer for such a
thing) seemed manageable, but a round trip seemed too expensive and too
difficult. To date, it has never been attempted, but the idea has been tempting
and there are plans to send people to Mars (it remains to be seen if and when
these plans will succeed).

Since Mariner 4, the U. S. has sent several spacecraft which either flyby or
orbit Mars: Mariner 6, 7 and 9, Viking 1 and 2, Pathfinder, the Mars Global
Surveyor (MGS) and Odyssey. Odyssey was launched in April 2001 and
entered orbit around the red planet in October 2001. These spacecraft along with several soviet spacecraft have
returned thousands of photographs and a vast quantity of other data. In
addition, a dozen or so meteorites are known to have originated on Mars.
Analysis of these meteorites has supplied additional data.

We now have a very different picture of Mars. Some parts of Mars have numerous
craters suggestive of Mercury and the Moon, but other parts of Mars have plains,
volcanoes, canyons and river channels. The volcanoes and canyons are bigger
than any other known examples, however there is a vague similarity between some
of these features and similar features on the Earth. There was no evidence of
canals or liquid water.
However data prove Mars was warmer and had abundant liquid water in its early history.
Today there is still water, but almost all is in the
form of ice in the polar caps and below the surface (some locations on Mars may
experience temperatures above the melting point of water,
hence transient pools of liquid water are possible). There is also the
possibility Mars may have had tectonic plates like the Earth does now (if so,
they were active for only a 500 million years or so).

We now know that the
atmosphere has a pressure that varies between 5 and 10 millibars (much lower
than anyone had suspected until Mariner 4 made radar occultation measurements).
It is almost entirely carbon dioxide, but contains some water vapor and other
trace gases. The polar caps are partly water ice and partly frozen carbon
dioxide, but there are differences between the northern and southern polar caps,
as there is between a polar cap seen in the Martian winter and a polar cap seen
in the Martian summer.

Since the canals are not real, why were Schiaparelli, Flammarion and Lowell
(among others) so convinced they were real? There are some clues. First, Schiaparelli was
colorblind and this may explain why he saw details others did not. Once
Schiaparelli’s results were known, the power of suggestion may have influenced
other observers. Also, records suggest most observations of canals happened
under poor seeing conditions or when small apertures were used. The canals
disappeared under better conditions and larger apertures. Lowell preferred to
reduce the aperture of his scope (which made observing the canals easier), but
many of his critics used larger apertures.

There also have been a few tantalizing clues suggestive of life, but to date no
proof that Mars has or ever had life. The most publicized of these clues was a
meteorite that was given the designation ALH84001. ALH84001 is one of the dozen
or so meteorites known to come from Mars and had what looked like fossils. Some
scientists believe these fossils come from ancient Martian bacteria, however
other scientists are not convinced. I should note that Viking photographs in
the region known as Cydonia look like a human face, but MGS photographs of the
same region look like a pile of rocks. A few non-scientists claim this is a
structure built by Martians, however that is unlikely.

There is currently a spacecraft enroute to Mars; it was launched by Japan
in 1998. There were some technical problems, but it is expected to arrive at
Mars in late 2003.

Anyone with a telescope can attempt to observe Mars themselves. The best time
to observe Mars is the couple months before and after opposition (the next opposition is in the
year 2003). The rest of the time, it is difficult to see
any detail. Every 15 years there is an exceptionally good opposition; the last
one was in 1988, the next one is in 2003.

Observing Mars takes practice. Details become clear after a little acclimation.
If the seeing is bad, you will not observe as much detail as when the seeing is
good so patience is important. You should try to observe Mars as often as
possible during the opposition, this will allow you to track changes in surface
and atmospheric features. When you observe Mars, you may want to try sketching;
this will train your eye to observe detail. Generally the polar caps are the
easiest features to see, however you should see the maria and deserts as well.
If you observe over long periods and are patient, you may see clouds, dust
storms and various atmospheric phenomena. You may also notice changes in the
polar caps and the maria.

If you have a good telescope and sharp eyes it may be possible to see the two
moons, Phobos and Deimos. At best they have magnitudes 11 and 12, and are
rather close to the bright red Mars.

Observers have seen various types of clouds on Mars. They are known by the
labels blue, white, yellow and W-shaped. These labels can be misleading.
Yellow clouds look yellow to the eye, however blue clouds do not necessarily
look blue, white clouds do not necessarily look white and W-shaped clouds are
not always W shaped. Yellow clouds are composed of dust and sometimes grow to
cover much of the Martian surface, when this happens it is known as a dust
storm.

Having the correct equipment will help your observations. If you wish to
observe surface details, a dark yellow, red and/or orange filter is helpful.
Violet and blue filters are helpful if you want to observe clouds and other
atmospheric phenomena (but not yellow clouds or dust storms). Green filters are
helpful for observing the polar caps and other white areas, yellow clouds and
dust storms. If you have made either Jupiter or Saturn observations, you may
want eyepieces that provide slightly more magnification than the eyepieces you
used for Jupiter and Saturn.

One phenomenon worth mentioning is the violet clearing. When Mars is observed
through a blue or violet filter, it usually appears as a featureless blob (but
clouds can sometimes be observed). However on occasion (usually only once every
few years) details on the surface appear. This lasts a few days; such events
are known as violet clearings. It has been suggested this demonstrates a poorly
understood change in the Martian atmosphere, but the best evidence suggests it
has nothing to do with the atmosphere at all and is probably an optical
illusion.

For more information

The Mars photo above is from the Hubble Space Telescope
Wide Field Planetary Camera-2. It was taken on March 10, 1997,
just before opposition and just before summer solstice.
The white area at the top of the photo is the permanent north polar cap.
A haze can be seen in the equatorial
region. The dark area near the center of the photo is Syrtis Major.